The invention relates more particularly to a pneumatic servomotor for power-assisted braking for a motor vehicle, which servomotor is: of the type having a rigid casing inside which a moving transverse partition is mounted to define in leaktight manner a front chamber subjected to a first pressure, and a back chamber subjected to a second pressure varying between the first pressure and a pressure that is greater than the first pressure, which partition can drive an actuating rod for actuating a master cylinder associated with the servomotor via a reaction disk, and is returned resiliently by first return means; of the type having a control rod moving inside the piston selectively as a function of an axial inlet force exerted forwards against a return force applied to the rod by second return means; of the type in which the movements of the control rod can determine opening and closing of at least one axial valve referred to as an “inlet” valve and which is interposed between a pressure source subjected to the pressure greater than the first pressure and the back chamber, and at least one axial valve referred to as a “balancing” valve and which is interposed between the front chamber and the back chamber, for actuating the above-described moving partition; and of the type in which a plunger passing through the moving partition and secured to the end of the control rod can drive the actuating rod of the master cylinder directly, via the reaction disk.
Numerous examples of conventional servomotors of this type are known.
In such a servomotor, a piston is secured to the moving wall into which it is fitted, for example, and it receives the inlet and balancing valves. In addition, the balancing valve and the inlet valve are part of a single three-port valve in which a common seat is constituted by a shoulder front face of a moving element which is resiliently returned against a collar on the plunger and against a collar on the piston, from which collars it can be selectively separated to open the balancing valve or the inlet valve.
Thus, a substantially radial duct that passes through the piston and that opens out into the front chamber, and a substantially axial duct which opens out to the outside of the servomotor are selectively put into communication via a respective one of valves with a radial duct that passes through the piston and that opens out in the back chamber to maintain or to reduce to zero a pressure difference between the back and the front chambers, and to cause the moving partition to move.
Such a design suffers from the drawback of requiring ducts to be formed in the piston with complex shapes, which disturbs the flow of air. As a result, a servomotor generally has response times that are relatively high, and is noisy.
In addition, such a design is particularly costly to implement because it requires a piston to be used whose internal shapes, serving to form the ducts, can be formed only by performing a machining method.